Certain properties of gypsum make it very popular for use in making industrial and building plasters and other building products, especially gypsum wallboard. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed to useful shapes. It is also noncombustible and relatively dimensionally stable when exposed to moisture. However, because it is a brittle, crystalline material, which has relatively low tensile and flexural strength, its uses are typically limited to non-structural, non-load-bearing and non-impact-absorbing applications.
Gypsum wallboard, also known as drywall, consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets and is used largely for interior wall and ceiling applications, as well as other specialty uses. The paper cover sheets contribute significantly to the strength of wallboard; without the paper up to 80% of the flexural strength is lost. Furthermore, because of the brittleness and low nail and screw holding properties of its gypsum core, conventional wallboard by itself cannot support heavy appended loads or absorb significant impact.
A material which may be used to make a wallboard having numerous properties superior to those of paper-covered gypsum wallboard is taught in a patent assigned to United States Gypsum Company, U.S. Pat. No. 5,320,677 (the ""677 patent). The ""677 patent teaches a gypsum composite material which is strong enough so as not to require a paper covering. The gypsum composite material is made from a combination of gypsum and a xe2x80x9chost particlexe2x80x9d such as a lignocellulosic material. This combination produces a composite material useful for making strong cast products, plasters, building products and for other applications. The composite material may be used to make a paperless gypsum wallboard also known as xe2x80x9cfiberboardxe2x80x9d. This fiberboard is fire resistant, has non-directional strength, including resistance to nail and screw pull-out throughout its expanse, and a harder finish than paper-covered wallboard. It also has better dimensional stability than paper-covered wallboard and has the ability to maintain its strength even in a humid environment. In addition, fiberboard has fewer defects related to the use of paper, e.g., problems such as the tearing of the paper covering or bubble formation under the paper are eliminated. Finally, fiberboard can be produced at a competitive cost.
A disadvantage of the fiberboard of the ""677 patent is that the surface is not as smooth as would be desirable, because of the inclusion of the fibrous lignocellulosic material. A smooth, flat and level surface is preferred for applications such as ceilings and wall assemblies. Current methods used to smooth the rough surface of fiberboard and to minimize the surface imperfections may involve many laborious steps and add significant expense to fiberboard production. Rough-surfaced materials may be covered with conventional paints or coatings to help hide the imperfections, but because these coatings tend to shrink as they dry, the finished surface follows the contours of the rough surface and a flat, level, and smooth surface does not result. Rough-surfaced materials may also be sanded or polished after coating to yield a smooth surface and multiple coating/sanding operations may be needed to produce a smooth surface. Multi-step coating and sanding (or polishing) operations have been used to prepare the surface of fiberboard substrates such as medium density fiberboard. At each coating step, the wet coating must be driedxe2x80x94a process that involves energy and requires physical space on a production line. Multi-step surface preparation procedures require much longer production lines and increase energy costs, as well as adding expense for sanding or polishing materials, and for disposal of waste materials.
If the fiberboard could be coated with a material that does not shrink as it dries, then the rough surface could be covered, leaving a flat, level and smooth surface. It would be possible to make a smooth, flat-surfaced fiberboard in fewer surface preparation steps, e.g., in only one or two steps without sanding or polishing.
In principle, the fiberboard could be coated with calcium sulfate hemihydrate (plaster of Paris), which will convert to calcium sulfate dihydrate (gypsum) when mixed with water, resulting in hardening or setting of the plaster. The reaction inexorably proceeds to completion in a period of time as adjusted by various accelerators and retarders, usually 5-300 minutes after mixing with water. A typical calcium sulfate hemihydrate coating would thus need to be kept in a dry form, then mixed with water just before being used to coat the fiberboard. But, the machinery used to coat the fiberboard would have to be frequently cleaned to remove accumulation of calcium sulfate dihydrate. Thus, using a coating based on calcium sulfate hemihydrate using conventional set control additives is not amenable to a continuous in-line production process. But, if the gypsum composition did not set when mixed with water, constant cleaning of the machine would not be needed. Then, if the gypsum composition only set after being coated on the fiberboard panel, then the desired smooth finish on the rough panel could be achieved.
The present inventors have discovered how to produce a smooth, flat gypsum-coated fiberboard. Their invention includes a paperless fiberboard with a surface acceptable for wall and ceiling applications and a process for making such fiberboard.
The present invention is a process for preparing a smooth-surfaced gypsum-coated substrate from a rough-surfaced substrate, especially from fiberboard that previously could not be smoothed in an economical manner and the resulting smooth-surfaced substrate.
In one embodiment of the present invention, a rough-surfaced fiberboard substrate is made smooth by the application of a calcium sulfate hemihydrate-containing coating composition with a reverse roll coater. As the fiberboard is fed through a reverse roll coater, a layer of the coating composition is deposited onto the surface of the board where hydration occurs to set the coating. The surface of the finished substrate is smooth, flat and level at the time the fiberboard emerges from the reverse roll coater. Since the coating composition contains a set preventer, it does not set until after deposited on the surface of the fiberboard where an activator initiates the setting reaction only on the panel and not in the coating contained within the reverse roll coater, thus permitting the use of a reverse roll coater for continuous operation. A continuous operation would not be possible using conventional set controlled calcium sulfate hemihydrate coatings, as the setting reaction would proceed inexorably to completion once mixed with water, necessitating shut down of the equipment to clean out the set gypsum in and on the roll coater.
In a preferred embodiment, the set preventers are the non-calcium-bearing phosphate compounds of U.S. Pat. No. 5,746,822. After coating the fiberboard substrate with the coating composition, and the coating composition sets, i.e., the calcium sulfate hemihydrate in the coating composition hydrates to form calcium sulfate dihydrate (gypsum). In order for this setting process to occur, a set initiator must be present, either in or on the surface of the fiberboard, or alternatively introduced into the coating as it is applied. Set initiators include the accelerators of the ""822 patent. While it was suggested in the ""822 patent that aluminum sulfate and ferrous ion-containing compounds could be used, they were not recommended. However, aluminum sulfate is a preferred set initiator in the present invention. The amount of the set initiator will depend on the amount required to overcome the set preventing effect of the non-calcium-bearing phosphates. Generally, about 0.01 to 0.03 wt % of the set initiator, based on the weight of the coating composition, will be sufficient. Using a very active set initiator such as aluminum sulfate has the advantage of rapidly hydrating the calcium sulfate hemihydrate so that it does not need to be mixed with the coating composition, but may be merely on the surface of the fiberboard.
Where the rough-surfaced substrate to be coated is a gypsum-containing fiberboard material, smoothing the substrate will produce a product having the beneficial properties of the rough-surfaced fiberboard material (uniformly good strength, including resistance to nail and screw pull-out throughout its expanse, dimensional stability, maintenance of strength even in a humid environment, fire resistance, and low cost) and a smooth surface which is amenable to decorative finishes. The surface should be similar to a troweled plaster in its hardness and durability.
Description of Illustrative Embodiments Coating Compositions
The coating composition contains calcium sulfate hemihydrate, but it does not set to form gypsum, even though mixed with water. Thus, continuous production of coated fiberboard is possible. A calcium sulfate hemihydrate-containing coating composition that can be premixed with water off-site and brought to a production line ready for use. Unlike conventional calcium sulfate hemihydrate compositions, the inventive coating composition will not inexorably proceed to set when mixed with water, but the calcium sulfate hemihydrate remains unset indefinitely, even though mixed with water. When desired, the coating composition can be caused to completely set and harden (thereby converting the hemihydrate to calcium sulfate dihydrate) upon contact with a set initiator. In this manner, a gypsum-coated substrate may be created in a continuous production line environment without the need for on-site mixing of a coating composition with water and without the need for using the coating composition immediately after mixing the calcium sulfate hemihydrate with water. Furthermore, because the coating composition will not set unless a set initiator is present, it will not set on the production equipment and thus the equipment does not require removal of set gypsum and may be readily cleaned after use. The wet-mixed coating composition of the present invention can be added into a reverse roll coater and maintained in the unset (calcium sulfate hemihydrate) state until it is desired to initiate the setting reaction. The coating composition of the present invention starts setting only after it had been applied to the substrate. This permits a smooth, flat and level substrate surface to be formed with a minimum of processing steps.
Conventional non-calcium sulfate hemihydrate coatings tend to shrink after application to a substrate as they dry. On shrinking, they follow the contour of the surface and, if the surface is rough, many coats and sanding steps may be needed to yield a smooth, flat substrate. With the present coating composition, there is substantially no shrinkage of the coating as it dries. The coating is thick enough, preferably about {fraction (1/32)} to {fraction (1/16)} inches (0.79 to 1.59 mm) to conform to the flat level surface of the reverse roll coater and to cover the rough surface of the fiberboard, thus generating the desired smooth, flat, level substrate.
Non-Calcium Bearing Phosphate Set Retarders
As discussed in the ""822 patent, certain non-calcium bearing phosphates have been found to have a long term set preventing effect. The present inventors believe that set prevention better describes the performance of these phosphates, rather than merely xe2x80x9cset retarders.xe2x80x9d Examples of such phosphates include zinc hexametaphosphate, potassium tripolyphosphate, tetra sodium pyrophosphate, sodium tripolyphosphate, monoammonium phosphate, and monobasic potassium phosphate. Tetra sodium pyrophosphate is a preferred set preventer. The amount required may vary from about 0.1 to about 10 wt. % based on the total solids content of the coating, preferably from about 0.2 to about 0.5 wt. %.
Other Additives
The non-calcium bearing phosphate compounds are needed to prevent the calcium sulfate hemihydrate from prematurely setting when in the presence of water. Stiffness is provided by the addition of insoluble particulates, such as clays e.g. sepiolite, bentonite, or attapulgus clays, and mica. The term xe2x80x9cstiffnessxe2x80x9d implies a degree of thixotropic performance useful in the coating compositions. That is, the coating composition is relatively immobile until applied and worked with tools. In addition, the stiffeners add to the workability, and provide a suspending property that maintains a homogeneous mixture and keeps components from settling and prematurely forming an unusable and unworkable mass. Other additives may be included in order to provide other desired properties.
Conventional cellulosic thickeners may be added to adjust the viscosity of the coating composition. They generally do not provide the stiffening property available from clays and mica. Examples include ethylhydroxy ethylcellulose, hydroxypropyl methylcellulose, methylhydroxypropyl cellulose, hydroxyethyl cellulose, and the like. The amount used may vary up to about 1 wt. % of the solids content of the coating as required to meet the desired consistency. Preferably, about 0.35 to about 0.6 wt. %, based on the solids content of the coating composition, will be used.
Latex emulsion binders may be included in coating composition, of the invention. Examples include polyvinyl acetate and ethylene vinyl acetate emulsions. The amount used may range from about 1.5 to about 7 wt. % of the solids content of the joint compound, preferably about 2 to about 5.5 wt. %.
Reduced weight may be provided by addition of lightweight spheres, such as an expanded perlite or resin microspheres. Fibers may also be added, if desired.
Other additives which may have use in joint compounds of the invention include talc, citric acid, fungicides, and bactericides. Wetting agents, defoamers, and plasticizers may also be included.
Set Initiators
Set initiators, also called accelerators in the ""822 patent, contain at least one compound which has a log K value greater than calcium. Log K is the logarithm of the equilibrium constant K of the chelating reaction of diethylene triamine pentaacetic acid with a given metal cation. Table I in the ""822 patent provides a list of metal ions and their log K values. Zinc sulfate was considered to be a superior set initiator, although the numerical difference between zinc and calcium in Table I was not the largest shown. While it was suggested in the ""822 patent that aluminum sulfate and ferrous ion-containing compounds were not recommended since they have detrimental effects, aluminum sulfate is a preferred set initiator in the present application. The amount of the set initiator will depend on the amount required to overcome the set preventing effect of the non-calcium bearing phosphates. Generally, about 0.01 to 0.03 wt. % of the set initiator, based on the weight of the premixed first part of the coating compound, will be used. It has been found that a very active set initiator, such as aluminum sulfate, does not have to be mixed with the coating compound, but by merely being on the surface of the substrate, causes the setting reaction to occur throughout the coating.
Although the reasons for the effectiveness of the set preventers described above are not fully known, it is believed that they function by sequestering calcium ions and/or calcium compounds, inhibiting crystal growth. It is believed that the set initiators function by preferentially binding to the set preventer, freeing the calcium ions and/or calcium compounds to initiate the setting reaction and crystal growth.
When set initiators are used with ready-mixed setting-type joint compounds, it is typical that they are mixed thoroughly with the ready-mixed joint setting-type compound just prior to use. A colorant may be included to help assure complete mixing. In the present use, the ready-mixed coating composition could be mixed with a set initiator as it is applied, but preferably, the set initiator is included in the gypsum formulation used in making the fiberboard or is applied to the top of the fiberboard as it is curing. It has been found that aluminum sulfate is sufficiently active so that its presence on the surface of the fiberboard is sufficient to begin the setting process, which then proceeds throughout the coating layer.
Typical coating compositions of the present invention are represented by the formulations listed in Table 1.
Fiberboard
A rough-surfaced fiber-containing substrate may be made smooth by the application of the calcium sulfate hemihydrate-containing coating composition described above. Such a fiber-containing substrate may be prepared by the method taught in U.S. Pat. No. 5,320,677 (Baig) assigned to United States Gypsum Company. In the ""677 method, host particles are used to strengthen gypsum in the preparation of the fiber-containing substrate. The host particles that may be used are any macroscopic particle such as a fiber, chip or flake of a substance other than gypsum. The particle is generally insoluble in the gypsum-containing slurry and may also have accessible voids on the surfaces such as pits, cracks, fissures, hollow cores or other surface imperfections which are penetrable by the slurry and within which calcium sulfate crystals can form. A lignocellulosic fiber, particularly a wood fiber, is an example of a host particle especially well-suited for the composite material and process of this invention. Another readily available and affordable material that is also widely used in building products is lignocellulosic material, particularly wood and paper fibers. Particle board, fiberboard, waferboard, plywood and hardboard (i.e., high density fiberboard) are some of the forms of processed lignocellulosic material products used in the building industry.
To prepare the fiber-containing substrate, uncalcined ground gypsum and the host particles are mixed together with sufficient liquid to form a dilute slurry consisting of at least about 70% by weight liquid. The mixture is then heated under pressure to calcine the gypsum, converting it to acicular calcium sulfate alpha hemihydrate crystals. The material can be dried immediately before it cools to provide a stable, but rehydratable hemihydrate composite for later use. Alternatively, if it is to be directly converted into a usable product form right away, such as fiberboard, the composite can be further separated from substantially all the liquid except that needed for rehydration, combined with other like composite particles into a desired shape, and then rehydrated to a set and stabilized gypsum composite mass. An optional step in the process is the continuous agitation of the slurry while the gypsum is being calcined and the hemihydrate crystals are forming.
As described in the ""677 patent, after the conversion of gypsum to the hemihydrate has been completed, the slurry of hemihydrate and fibers may be discharged onto a dewatering conveyor and cooled and pressed to form a fiberboard while rehydration to gypsum is occurring. Various additives may have been added to the slurry, including aluminum sulfate, which serves as an accelerator and also serves as a set initiator for the coating composition when it is applied to the fiberboard surface.
Application of the Coating
While in principle any method could be used to apply the coating to the surface of the fiberboard, for commercial applications coating equipment designed for that purpose will be needed. One such is the reverse roll coater, which deposits the coating and uses a roller to spread the coating as the roller turns in a direction opposite to the movement of the fiberboard conveyer.
One example of a reverse roll coater is made by Black Bros: Model 222 Reverse Roll Filler. The equipment is designed to apply various types of coatings to sheet materials. The sheet material is transported by feed rolls below, while the coating is applied by a coating roll to the upper surface. The coating roll receives the coating material from a doctor roll which meters the coating. While the coating roll rotates in the same direction as the sheet material is moving, the excess coating is removed by a wiping roll rotating in the opposite direction (i.e. a reverse roll). Lower rolls support and feed the sheet material at a constant speed while a wiping blade removes and recycles the excess coating.